The present assignee has developed a printable LED light sheet where microscopic inorganic LED dies, having a top electrode and a bottom electrode, are printed as an ink on a conductive layer on a thin substrate. Such LEDs are called vertical LEDs. The ink comprises the LED dies uniformly infused in a solvent. After the ink is cured, the bottom electrodes of the LEDs make electrical contact to the conductive layer. A dielectric layer is then deposited between the LEDs, and another conductive layer is printed to make electrical contact to the top electrodes of the LEDs to connect the LEDs in parallel. A suitable voltage is applied to the two conductive layers to illuminate the LEDs. To allow light to escape, one or both of the conductive layers is transparent. Indium tin oxide (ITO) or sintered silver nano-wires may be used for the transparent conductive layer. Other conductive oxides may also be used. Such a technique is described in the assignee's US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of a Liquid or Gel Suspension of Diodes, incorporated herein by reference.
One possible application of the light sheet technology is for addressable displays. However, since the printing process inherently prints the LEDs in a random pattern, there must be some means for printing separately addressable groups of LEDs. This may be done with screen printing, flexography, inkjet printing, or other printing method where the ink is dispensed as small, electrically isolated dots (pixels) over the substrate. However, it is difficult to form identical small pixels in this manner, and the deposited ink spreads somewhat. Further, when the groups of LEDs (pixels) are addressed and energized to create a lit pixel, the light spreads into adjacent pixel locations, reducing contrast and resolution.
What is needed is a technique for forming an addressable display using printed LEDs where pixels can be precisely formed and where there is reduced cross-talk between pixels.